Pressure sensor

ABSTRACT

A sensor for detecting localized pressure bearing on the sensor, comprises a volume of compressible, light translucent material having light-scattering centers evenly dispersed throughout the material. A light emitter communicates with the material to form a virtual optical cavity within the material; the intensity of scattered light is detected by a detector. A second detector is positioned at some remove from the first detector, outside the virtual optical cavity. Pressure bearing on the material increases the intensity of scattered light within the optical cavity. The decreased intensity is measured by the second detector. A signal processor receives signals from the first and second detectors, and resolves these signals into information indicative of pressure bearing on the sensor. The light&#39;s scattering sensors may comprise particles coated with a thermal chromic substance, for detecting both temperature and pressure

FIELD OF THE INVENTION

[0001] The present invention relates to an improved sensor for detecting localized pressure, for a wide variety of applications.

BACKGROUND OF THE INVENTION

[0002] Sensors for the detection of localized pressure (i.e. pressure imposed on a particular object, as distinct from atmospheric pressure) may employ the principle whereby the intensity of light or other wave energy from a source, which is diffused and scattered within a scattering medium such as translucent foam, is increased in the vicinity of the light source as the concentration of scattering centers within the medium increases, i.e., the average distance between scattering centers decreases. A similar effect is achieved if the nature of the scattering centers changes to change their light reflective or refractive properties.

[0003] The region within the medium which contains scattered light from the source is known as a “virtual optical cavity”, since this region effectively emulates an optical cavity. For simplicity, this region will be referred to as simply an “optical cavity”. The intensity of diffused or scattered light at any particular position within an optical cavity is referred to as the “integrated intensity” of the light at that position. Thus, as the medium is compressed by the application of pressure, the integrated intensity of the light within the region immediately surrounding the light source increases in intensity. The increase is proportional to the increase in concentration of scattering centers or a change in the nature of the scattering centers. This in turn may be related to increases in localized pressure applied to the medium. The consequent decrease in light intensity occurs within a more distant region within the medium. For example, U.S. patent application Ser. No. 08/895,268 (Reimer et al.) describes a pressure sensor based on this principle, in which the scattering medium may comprise either a material having scattering centers dispersed generally evenly therein, or a hollow deformable container, the inner surface of which diffuses light or other wavelike energy directed into the medium. The light source forms an integrated cavity within the medium, defined by a region containing fully scattered light from the source. When pressure is applied to the medium, the medium compresses and increases the concentration of scattering centers in the region surrounding the light source. The resulting increase in light intensity is detected by a receptor and communicated to an information processor. In one version, a multiplicity of light sources and receivers permits the general location of the pressure to be resolved. Within an apparatus of this type, one or more light sources and detectors are provided, with each the source and a corresponding detector being generally adjacent to each other or close together. Most conveniently, the scattering medium comprises a compressible, translucent material such as plastic foam. An array of source/detectors pairs may be provided to provide localized pressure detection means. The detector or detectors are associated with a signal processing unit, which receives information from the detectors corresponding to the detected integrated light intensity levels, and resolves this information into a corresponding pressure level experienced by the scattering medium.

[0004] References herein to the word “light” includes within its scope light in visible and non-visible wavelengths.

[0005] The pressure sensors of the type characterized within the above-referenced prior art, may be subject to “noise” as a result of several factors. Most importantly, interference may result in a change in the light absorption characteristics of the scattering medium, or the scattering centers themselves. A change in absorption would effect light intensity within the regions surrounding the light source, and this could be mistaken for a deformation effect. Such a change might take place in a polymeric medium as the result of long term aging photo-oxidation. It would therefor be valuable to provide more robustness to this type of sensor by enabling it to better differentiate noise from signal. It is accordingly desirable to integrate within a sensor of this sort, a means to measure the light absorption properties of the scattering medium. Absorption measurement for optical energy or other forms of directly transmitted or reflected wave energy is a well known art, and the principles for the measurement for absorption in transmission or reflection by means of various photometers have been thoroughly documented in scientific literature. However, it has not been previously proposed to introduce an absorption measuring element into a pressure sensor of the above type.

SUMMARY OF THE INVENTION

[0006] An object of the invention is to provide a pressure sensor for detecting point source or localized pressure, which operates by detection of the intensity of scattered light within a compressible carrier medium, and which has an improved signal to noise ratio.

[0007] In one aspect, the invention comprises a pressure sensor having improved signal to noise sensitivity, of the type comprising a volume of a generally translucent material having light scattering centers evenly dispersed therein, and which is readily deformable under pressure;

[0008] a source of light or other wave energy associated with the material and positioned to direct light into the material to form a virtual optical cavity within the material within which light from the source is fully scattered;

[0009] a first light detector, in operative association with the translucent material positioned generally adjacent to or in the immediate vicinity of the light source, for detecting light intensity within the optical cavity; characterized by:

[0010] a second light detector in operative association with the translucent material, for detecting the intensity of light within the material at a position outside the optical cavity; and

[0011] signal processing means operatively associated with the two detectors, for receiving light intensity information from the detectors, and resolving the information thus received into a measure of localized pressure bearing on the material.

[0012] In the above aspect, the invention takes advantage of the phenomenon whereby deformation or compression of a material having light scattering centers dispersed therein causes an increase in the integrated intensity of light emanating from a point source within an optical cavity formed by the light within the material. The increase is proportional to a decrease in light intensity within the material at positions immediately outside the optical cavity. By measuring scattered, or integrated, light intensity both within the optical cavity and immediately outside the cavity, an enhanced sensitivity and improved ability to discriminate changes in light scattering from changes in absorption, may be achieved over measurement only of light intensity within the cavity. The results apply both with respect to measurement and for purposes of noise/signal discrimination.

[0013] Having thus generally characterized the invention, a detailed description of preferred embodiments of the invention will follow, by way of reference to the attached drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view of a prior art pressure sensor;

[0015]FIG. 2 is a further schematic view of a prior art pressure sensor, in a compressed position;

[0016]FIG. 3 is a schematic view of a portion of a first embodiment of a pressure sensor according to the present invention;

[0017]FIG. 4 is a graph illustrating the signals transmitted by a first embodiment pressure sensor, in response to deformation of the sensor;

[0018]FIG. 5 is a second embodiment of a pressure sensor;

[0019]FIG. 6 is a graph illustrating the signals transmitted by the second embodiment pressure sensor in response to deformation;

[0020]FIG. 7 is a schematic view of a portion of further embodiments of a pressure sensor;

[0021]FIG. 8 is a graph illustrating the signal transmitted by the embodiments of FIG. 7, in response to temperature;

[0022]FIG. 9 is a graph illustrating a signal transmitted by the sensor of FIG. 7 in response to pH levels within a medium exposed to the sensor;

[0023]FIG. 10 is a graph illustrating the signal transmitted by the sensor of FIG. 7, in response to levels of selected ions within a medium;

[0024]FIG. 11 is a graph illustrating the signal transmitted by the sensor, in response to levels of specific biological antigens or antibodies within a medium;

[0025]FIG. 12 is a schematic view of a portion of the invention illustrating further embodiments thereof;

[0026]FIG. 13 is a graph illustrating the signal transmitted by the embodiment of FIG. 12, in response to levels of radiation exposed to the sensor;

[0027]FIG. 14 is a graph illustrating the signal transmitted by the embodiment of FIG. 12, in response to an electric field;

[0028]FIG. 15 is a schematic view of a further embodiment of the invention;

[0029]FIG. 16 is a graph illustrating the signal transmitted by the embodiment of FIG. 15, in response to pressure and temperature detected by the sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention relies on the use of a volume of material which is translucent to light and under at least certain conditions contains light-scattering centers which fully scatter light entering the material. A prior art pressure sensor which relies on this principle is illustrated in FIGS. 1 and 2. A sensor of this type is characterized by a scattering medium 5, formed from a deformable compressible material having evenly dispersed therein a plurality of scattering centers. For example, the material may comprise a translucent cellular foam material. A light emitter 4 and detector 6 are positioned within the interior of the material. Conveniently, the light source and detector may comprise fiber optic cables, the free end of which terminates within the interior of the material. The emitter/detector pair are adjacent to each other or spaced apart by a spacing in the order of several millimeters. The light emitter 4 illuminates a region 7 within the material, by illumination having a characteristic intensity level. The size of region 7 is determined by the nature of the scattering material, as well as the intensity of light emitted by the light emitter 4. It will be further seen that any appropriate source of wave energy (i.e. having a suitable output spectrum) may be transmitted into the scattering medium, including electromagnetic radiation within the non-visible spectrum. The nature of the scattering medium will be determined according to the nature of the wave energy.

[0031] The light emitter/detector pair communicates via fiber optic cables 1 and 2, with a light source 9 and photoreceptor 11, respectively. The light source 9 may comprise an LED or any other appropriate light source having a suitable output spectrum. The photoreceptor 11 comprises any conventional light detection means which emits electronic signals responsive to light levels. The light source and photoreceptor respectively both communicate with a signal processing unit 10, which powers the light source, and also translates and resolves the information received from the photoreceptor 11, into a measure of the pressure bearing on the detector. The CPU 10 communicates electronically in turn via a power and data connection line 12, with a downstream receiver such as a display means or the like (not shown).

[0032] The scattering medium 3 conventionally forms a thin sheetlike member, bounded on its upper and lower surfaces by a protective layer 14, such as fabric.

[0033] Upon compression of the scattering medium as seen in FIG. 2, the scattering centers within the medium become more densely packed together. As a result, the region 7 effectively illuminated by the light source contracts by virtue of the increased density of the scattering centers. In consequence, the integrated light intensity within the region 7 will increase, and this increase is detected by the detector 6. The processing unit 10 in turn translates this information as an increase in pressure experience by the detector. The increase in light intensity is proportional to the deformation of the deformable material. Providing that the coefficient of the deformation of the material is known, the processing unit 10 is thus capable of providing a reading of the pressure experienced by the deformable material.

[0034] While the illustrated prior art version shows a single emitter/detector pair, it is feasible to provide multiple, spaced apart emitter/detector pairs to provide a measure of localized pressure bearing on the detector.

[0035] The effectively illuminated region 7 within the scattering medium is referred to herein as a “virtual optical cavity” or for convenience simply an “optical cavity”. The optical cavity is characterized by a region within which light emitted by the emitter 9 is fully scattered and diffused. Within the optical cavity, the scattered and diffused light increases in intensity as the medium is compressed and the scattering centers are correspondingly concentrated. Light received by the detector is substantially fully scattered and is not received directly from the emitter. The size of the zone of effective illumination comprises the optical cavity, which is defined as radius of one characteristic scattering length of light within the medium. This in turn will depend on the scattering centre density or the nature of scattering centers within the medium. The cavity will decrease in volume as the medium is compressed and the scattering center density correspondingly increases.

[0036] It will be further seen that compression of the scattering medium, which results in a contraction of the size of the optical cavity and a corresponding increase in light intensity therein, also results in a corresponding decrease in light intensity within a region outside the optical cavity.

[0037] A first embodiment of the present invention is illustrated schematically in FIG. 3. In this version, the phenomenon whereby light intensity increases within the optical cavity upon an increase in concentration of the scattering centers, and correspondingly decreases outside the cavity, is harnessed to provide a pressure sensor having enhanced sensitivity. In this version, a deformable and compressible scattering medium 20 is provided, of the general type as comprised above. A relatively closely spaced apart emitter/detector pair 22(a) and (b) communicates with the scattering medium, for example, by means of paired fiber optic cables implanted within the medium. A second detector 24 is provided within the medium 20, at some remove from the emitter/detector pair. The spacing between the second detector and the light emitter will depend in part on the sensitivity of the detector, the intensity of the light emitted by the emitter, and the scattering properties of the medium, e.g., the concentration of light scattering centers. The second detector 24 is positioned outside the optical cavity 26 formed by the light emanating from the emitter 22(a).

[0038] Upon compression of the scattering medium, the integrated light intensity within the optical cavity 26 increases. A corresponding decrease occurs in the region immediately outside the optical cavity, within which the second detector 24 is positioned.

[0039]FIG. 4 illustrates a first signal (line “a”) received by the first detector in response to increasing compression of the sensor 22(b), and a second signal (line “b”) received by the second detector 24, in response to the compression. It will be seen that with increasing pressure, the first detector detects an increasing integrated light intensity, while the second detector detects a decrease of light intensity. Secondary lines a′ and b′ represent a proportionate decrease in signal strength lost to light absorption within the scattering medium. The processing unit receives the light intensity information from both detectors 22(b) and 24, and resolves same into a measure of the pressure bearing on the sensor.

[0040] The dual detectors of the first embodiment permit enhanced sensitivity of the detector, and a reduction in the interference that would otherwise be experienced. Typically, interference results from a change in the light absorption characteristics of the transmission medium or of the scattering centers. For example, this might occur because of degradation over time of a polymeric scattering medium. A change in absorption characteristics would affect light intensity within the optical cavity and could be mistaken for a deformation effect. The enhanced resolution provided within this version enhances the ability of the detector to differentiate this form of “noise” from “signal”.

[0041] A further embodiment of the invention provides an example of the variance in signal (i.e., increasing vs. decreasing scattered light level) received by the detector depending on the spacing of the detector from the emitter, as illustrated within FIGS. 5 and 6. FIGS. 5 and 6 illustrate a generally conventional pressure sensor 30 of the type characterized above, comprising a compressible medium 32 such as an open cell urethane foam, laminated to a silicon substrate 34. A light emitting source such as a diode 36 mounted on the substrate directs light into the compressible medium, thereby forming an optical cavity within the region around the light source. A photoreceptor 38 on the substrate is positioned at some remove from the light source. In one version, the spacing is within approximately 2 mm, and in a second version, the spacing between the source 36 and detector 38 is greater then approximately 2 mm. In other versions, the actual spacing will depend on the nature of the compressible medium and the light intensity emitted by the source. The emitter and detector mounted on the silicon type circuit board 34 both “look” in the same direction, with an overlapping field of illumination and field of view. Within the first positioning mode, the sensor is positioned within a “characteristic scattering length” of the emitter, this being a distance within which light intensity increases in response to compression of the medium. In the second mode described above, the sensor is mounted at a distance greater than the characteristic scattering length. The resulting signal received by the respective receiver positions is illustrated within FIG. 6. Integrated light intensity detected by the detector 38 positioned within the field of illumination increases in response to compression of the medium (line “c”), while in the second more removed position, signal strength decreases in response to compression (line “d”).

[0042] In a further aspect, a sensor for detecting changes in physical, chemical or molecular biological conditions described below may be provided based on the above principles. The detector of this type is illustrated schematically within FIG. 7, and comprises a solid or gel scattering medium 40, having associated therewith an emitter/detector pair 42(a) and (b), of the type described above. The relatively closely spaced-apart emitter/detector pair 42 is associated with processing means 44, of the type described above.

[0043] For detection of temperature, the scattering medium comprises a solid or liquid translucent material, and preferably comprises a solid material such as opal glass, polyethylene, or a transparent polymer such as PMMA, or a hydrated polymer gel such as polyacrylamide, with an embedded scattering agent such as titanium dioxide particles generally evenly disbursed throughout. The translucent material expands in response to increasing temperature, thereby reducing the concentration of the scattering centers dispersed with the material. A thermal coefficient cubic expansion of such materials is in the order of 10⁻³ to 10⁻⁵ per ° C., resulting in a corresponding change in the concentration of the scattering centers. The resulting perturbation will result in a corresponding change in the integrated intensity of scattered light within the optical cavity formed in the region around the light emitter. As the coefficients of expansion are relatively small, this type of sensor is advantages for wide temperature ranges, for example, a device fabricated through the use of silica optical fibers embedded in opal glass can be used to measure temperature over a range from about 0° C. to 500° C.

[0044] In order to achieve a greater degree of sensitivity, for use within a narrower temperature range, the scattering medium may comprise a material which undergoes a polycrystalline phase transition within the temperature range of interest. There exists a large class of hydrocarbon polymers which can be engineered to undergo polycrystaline phase transition over a specified temperature range, or polymer gels that undergo phase transition at a specified temperature range. Within this type of material, crystallization increases the light scattering properties of the material. Within the transition temperature zone, the characteristic scattering length of the material and therefor the dimensions of the effective optical cavity, will be relatively sensitive to temperature change. An increase in temperature, will cause a decrease in scattering crystal concentration, thereby decreasing the integrated light intensity within the optical cavity. A sensor constructed using a suitable such material may have a transition temperature range of 5° C. to 10° C., and will therefor will produce a relatively large signal in response to a small temperature change.

[0045]FIG. 8 illustrates the signal vs. temperature achieved by the two versions described above. Line “e” represents the signal decreasing in inverse relation to temperature. Line “e′” represents the inverse relation between the density of the scattering medium and increases temperature whereby increasing temperature acts to effectively decrease the density of the scattering centers.

[0046] In a further aspect, a sensor detects changes in the acidity level within a medium. In this version the configuration is the same as that shown in FIG. 7. However, the scattering matrix differs. In this version, the emitter/receiver pair 42 is embedded within a hydrated polymer gel matrix 40, having light scattering particulates such as titanium dioxide particles homogeneously dispersed and trapped within the gel. Functional groups on the gel are treated to respond to a pH range of interest. As the gel deforms in response to changes in pH, the scattering centre concentration changes, thereby changing the intensity of light within the optical cavity formed around the light emitter.

[0047]FIG. 9 represents the decreasing signal in response to increasing pH (line “f”) and the corresponding decrease in density of the medium (line “f”).

[0048] In a further version of this embodiment, the functional groups within the gel matrix may comprise groups sensitive to levels of a specific ion within a medium. FIG. 10 represents the increase in signal strength in response to increasing ion concentration (line “g”) and the corresponding increase in density in the scattering medium (line “g′”).

[0049] In a further version of the same embodiment, the functional groups within the gel may be sensitive to molecular biological molecules or materials within a medium. For example, the functional groups embedded in the gel may comprise a particular immune reagent, which binds to a particular antigen in a biological antibody/antigen binding process. Exposure of the gel to the specific antigen results in an antigen/antibody binding reaction. The antibody/antigen complexes form the scattering centres within the gel. As the antibody/antigen complexes form, the scattering coefficient of the gel increases, thereby increasing the integrated light intensity within the optical cavity surrounding the light emitter. FIG. 11 illustrates the signal increase corresponding with increasing antigen concentration (line “h”) and increasing scattering center concentration (line “h′”).

[0050] Within a further embodiment, electromagnetic radiation or an electric field may be detected. In this embodiment, illustrated within FIG. 12, a light scattering medium 50 is encased within a housing 52 which is transparent to radiation having the range of wavelengths of interest, but which is substantially opaque to wave energy within the range of the emitter/detector pair (for example, visible light). Scattering centres dispersed homogeneously within the medium comprise radiation sensitive particles, which change the optical scattering parameters within the optical cavity surrounding the emitter/detector 54(a) and (b) upon exposure to radiation. In a further version, the medium 50 is characterized such that ionizing radiation may be detected within a medium which does not have specific scattering centres dispersed therein. In this case, a specific reactant may be unnecessary as the radiation itself may be sufficiently energetic to damage the medium, causing fissures, dislocations and defects therein, which themselves form scattering centres within the optical cavity. The resulting integrated light intensity within the optical cavity detected by the detector 54(b), in response to increasing radiation intensity, is further illustrated.

[0051]FIG. 13 illustrates the signal increase proportionate to the radiation level (line “i”) and the proportionate increase in scattering center density within the medium (line “i′”).

[0052] In an other aspect of the embodiment of FIG. 12, the detector having the same general configuration as shown in FIG. 12 is intended to detect intensity of an electric field. In this version, the scattering medium 50 comprises a hydrated gel having evenly dispersed therein functional groups sensitive to the presence of an electric field. Field sensitivity causes the gel to shrink or swell. Scattering centres such as titanium dioxide particles are homogeneously dispersed with the gel. The resulting expansion or contraction of the gel results in a corresponding increase or decrease of light intensity within the optical cavity. In this version, the housing enclosing the gel is transparent to electric fields, but opaque to wave energy within the range of light emitted by the emitter.

[0053]FIG. 14 illustrates within this version the inverse relation between electric field and signal strength (line “j”) and the decreasing concentration of scattering centres in the medium (line j′).

[0054] In a further embodiment a sensor may detect both temperature and pressure bearing on the detector. In this version, shown schematically in FIG. 15, first and second detectors 60(a) and (b) are embedded within a scattering medium 62. The first detector 60(a) forms a part of a relatively closely spaced emitter/detector pair and is positioned within the optical cavity. The second detector 60(b) is positioned outside the optical cavity, at some remove from the emitter 64. The first detector 60(a) reacts positively to compression of the medium 62, in the conventional manner described above. The second detector 60(b) is positioned sufficiently distant from the optical cavity, to be independent of light intensity changes resulting from pressure bearing on the material. For this type of sensor, the scattering centres comprise particles coated with a thermochromic substance such as 4,4-bis (N,N-diethylamino) styrll-2,2-bipyridine. This compound is one of a class of organic substances commercially available as thermochromic inks from Chromophone Hallcrest and others. See also U.S. Pat. No. 5,480482. These compounds change their optical absorption characteristics in response to temperature changes. This results in a proportionate change in the integrated light intensity within the optical cavity, and a further corresponding change in the region immediately outside the optical cavity detected by the second receiver. In one version, the scattering medium 62 may comprise an open cell polyurethane foam, compressible by about 50% in the pressure range of 100 Pa to 10,000 Pa, coated with a thermal chromic paint sensitive in the temperature range from 35° C. to 40° C. This version would have a pressure/thermal sensitivity quite similar to human skin. FIG. 16 illustrates at line “k”, the integrated light intensity received by the first detector 60(a) in response to pressure bearing on the scattering medium. Line I′ represents signal detected by the second detector 60(b). Lines k′ and I′ corresponds to the change in the respective signals in response to a decrease in temperature. 

I claim:
 1. A sensor for detecting localized pressure bearing on said sensor, of the type comprising: a volume of light-translucent material having light-scattering centres generally evenly dispersed therein; a light emitter in operative communication with said material for emitting light into the interior of said material and thereby forming a virtual integrated optical cavity within the interior of said material, said cavity defined by a region within which light from said source is generally fully scattered; a first light detector in operative communication with said material for detecting the integrated intensity of scattered light within said optical cavity; signal processing means in operative communication with said light detector, for converting said integrated light intensity level detected by said first detector to a corresponding signal; characterized by: a second light detector in operative communication with said material for detecting the intensity of light within said material, at a position outside of and adjacent to said optical cavity, for detecting a decrease in integrated light intensity which is proportional to an increase in light intensity within said cavity generated by pressure bearing on said volume of material; and said signal processing means being in communication with said second detector for converting the light intensity detected by said first and second detectors into a signal corresponding to the local pressure bearing on said sensor.
 2. A sensor as defined in claim 1 , wherein said light scattering centres comprise particles coated with a thermochromic substance adapted to change its optical absorption characteristics in response to temperature changes, for detecting a combination of pressure and temperature.
 3. A method for detecting localized pressure bearing on a pressure sensor, of the type comprising: providing a volume of light-translucent material having light-scattering centres generally evenly dispersed therein, a light emitter in operative communication with said material for emitting light into the interior of said material, a first light detector in operative communication with said material for detecting the integrated intensity of scattered light within said optical cavity, and signal processing means in operative communication with said light detector; imposing localized pressure on at least a portion of said volume of material, thereby increasing the integrated intensity of light within said optical cavity; processing signals received from said light detector within said signal processing means, into a useable signal indicative of said pressure; characterized by: providing a second light detector in operative communication with said volume of material and said signal processing means, said second detector being positioned to detect the integrated intensity of light in a region of said material outside of said optical cavity and adjacent thereto; detecting a decrease in integrated light intensity which is proportional to the increase in integrated intensity within said optical cavity; and resolving signals from said first and second light detectors within said signal processing means to provide a measure of the pressure bearing on said sensor.
 4. A method as defined in claim 3 , further comprising the step of sensing temperature, comprising the further step of providing as said light scattering sensors, particles coated with a thermochromic substance capable of responding to changes in temperature by changing its optical absorption characteristics; and resolving signals received from said first and second detectors into pressure and temperature information, within said signal processing means. 